In recent years, among color display devices such as a personal computer or a television used to display an image, flat display panels including liquid crystal display panels (LCDs), field emission display panels (FEDs) and PDP have attracted an attention as a display device which enables a large-sized and thin-sized panel. In particular, the PDP has a remarkable advantage such as a fast response and a wide viewing angle.
The PDP has a structure such that a front plate and a rear plate are oppositely arranged and then the circumference thereof is hermetically sealed by sealing parts. Discharge gas, such as Ne, Xe and others, is enclosed within discharge spaces formed between the front plate and the rear plate.
The front plate includes a plurality of display electrode pairs made up of, in pairs, a sustain electrode and a scan electrode formed on a glass substrate in a stripe arrangement, a dielectric layer covering the display electrode pair, and a protective film covering the dielectric layer. The display electrode pair is made up of a transparent electrode, and a bus electrode made of metal material and formed on the transparent electrode.
On the other hand, the rear plate includes a plurality of address electrodes formed on a glass substrate in a stripe arrangement; an underlying dielectric layer for covering the address electrode; a barrier rib for partitioning discharge space for every address electrode and formed on the underlying dielectric layer; and a phosphor layer emitting red light, green light and blue light, which is formed on the underlying dielectric layer disposed between the barrier ribs, and at a side face of the barrier rib.
The front plate and the rear plate are oppositely arranged such that the display electrode pairs are perpendicular to the address electrode, and a discharge cell is formed in an intersection portion of these electrodes. The discharge cells are arranged in a matrix, and a pixel for color display includes 3 discharge cells having a phosphor layer of Red, Green and Blue colors. The phosphor layer is arranged toward the direction of the display electrode pairs. The PDP displays color images in such a manner that predetermined voltage is applied between the scan electrode and the address electrode, and between the scan electrode and the sustain electrode to create gas discharge, and then a phosphor layer is excited and radiated by ultraviolet rays produced by the gas discharge.
As to such a structure of PDP, there is required that the protective film has a better resistance to sputtering and a high secondary electron emission coefficient, etc., and, for example, as the protective film, the magnesium oxide (MgO) film is widely employed. The resistance to sputtering and the emission characteristics of secondary electron permit preventing the sputtering of the dielectric layer and reducing discharge voltage.
The protective film is formed using electron beam evaporation or plasma gun deposition and the like, the significant differences of characteristics between obtained films occurs according to deposition methods, deposition condition, etc. There is known that the characteristic of the secondary electron emission of magnesium oxide (MgO) film employed as the protective film is varied by impurities incorporated into during the deposition process. The reason is that if oxygen exists insufficiently within a film, or impurities, such as H2 molecular and the like originated from water (H2O) included in magnesium oxide or atmosphere gas, are incorporated into the film, combinations of Mg atom and O atom within the magnesium oxide (MgO) film are disordered. In other words, during such combination, uncombined bonds (dangling bond) occur, thus influencing the energy band of magnesium oxide (MgO) and in turn changing the emission state of the secondary electron.
In view of the above, in the case that magnesium oxide (MgO) as a protective film is formed by means of using electron beam evaporation, an example in which the protective film having better film characteristics can be stably manufactured by introducing the gases including Oxygen gas or Water (H2O) into an evaporation chamber and then controlling the partial pressure of various gases in a constant range, has disclosed (As an example, see Patent document 1).
However, in the known deposition apparatus as disclosed in Patent document 1, in which protective film is continuously formed while a substrate is conveyed, it is difficult to uniformly form atmosphere across an entire evaporation chamber. In particular, in the case that the gases including water (H2O) as impurity gas are introduced into the evaporation chamber, since influenced by a temperature distribution within the evaporation chamber, the decomposition of water (H2O) by electron beam and the like, water molecule existing within the chamber is undesirably distributed. Therefore, stable control of film quality can not be performed. As a consequence, the electron emission characteristics of the protective film can not be stably controlled, and thus the non-lighting of pixel or the malfunction of lighting in the PDP can occur.
[Patent Document 1] Japanese Patent Unexamined Publication No. 2005-50804